The classification directory of the description of the product "usually consists of the host computer, the power supply, electrocardiographic electrodes. The host part usually includes signal input part, amplification circuit, control circuit, display part, recording part and power supply part. The electrical signals from different parts of the body surface are detected by the electrodes, filtered, amplified, and converted to analog-to-digital to form ECG waveforms, and the location of the heartbeat is recognized according to the waveforms." Intended use "To measure the time variability between consecutive cardiac cycles. Heart rate variability metrics that can be analyzed include either time domain or frequency domain metrics. Time domain metrics typically include the standard deviation of the cardiac cycle (SDNN), the square root of the mean square of the difference between normal neighboring cardiac cycles (rMSSD), and the percentage of total beats that have a difference of more than 50 ms between adjacent R.R. intervals (PNN50). Frequency domain metrics typically include low-frequency power high-frequency bands (HF), low-frequency bands (LF), very-low-frequency (VLF), and ultra-low-frequency (ULF). "